1. Field of the Invention
The present invention relates to an ultrasonic image processing apparatus that obtains the motion state of a subject by using an ultrasonic image of the subject acquired with ultrasonic waves, and also relates to a method for processing an ultrasonic image.
2. Description of the Related Art
Objective and quantitative evaluation of functions of body tissues such as the myocardium of a heart is very important for diagnosis of the body tissues. For example, a method of acquiring image data of a heart with an ultrasonic imaging apparatus and quantitatively evaluating based on the image data has been proposed.
For example, a method of executing a pattern matching process on images and obtaining motion information such as displacement and strain of tissues has been proposed. As a specific example, for evaluation of the myocardium of a heart, ultrasonic waves are transmitted to the heart, and volume data that represents the heart is acquired in each cardiac phase. Then, the pattern matching process is executed on the volume data, whereby the position of a local site of the myocardium is obtained in each cardiac phase and the motion of the myocardium is tracked. Based on the result of the tracking, wall motion information such as the motion vector of the myocardium and strain of the myocardium can be obtained. Then, on an MPR image generated from the volume data of the heart, the wall motion information is superimposed and displayed. In conventional techniques, a short-axis image in a short-axis cross-section of a heart is generated, and a color corresponding to the magnitude of wall motion information is assigned onto the short-axis image and displayed thereon. For example, on the short-axis image, motion information in the wall-thickness direction of the myocardium is superimposed and displayed.
Further, in the conventional techniques, a central axis is set for the myocardium represented in the volume data, a desired point is designated on the central axis by an operator, a plane is set from the designated point toward the myocardium, and an image with the plane expanded is generated and displayed (US Serial No. 2006/0291705). By this method, a plane that intersects the myocardium is set and, by generating an image on the plane, an image that represents the cross-section of the myocardium is generated.
However, the short-axis image according to the conventional technique is an image on a plane that obliquely intersects the myocardium. Thus, the short-axis image represents not the actual morphology of the myocardium but a tissue on the plane that obliquely intersects the myocardium. To be specific, a heart is a 3-dimensional object with curvatures changing spatially, so that when an arbitrary plane intersecting the heart is set, the plane is set obliquely with respect to the wall-thickness direction of the myocardium. Therefore, the short-axis image on the arbitrary plane represents the tissue on the plane set obliquely with respect to the wall-thickness direction of the myocardium.
Thus, the thickness direction of the myocardium represented in the short-axis image of the conventional technique does not necessarily coincide with the direction of a vector in the actual wall-thickness direction of the myocardium, and the thickness of the myocardium represented in the short-axis image is represented larger than the actual wall thickness. Wall motion information is motion information in the actual wall-thickness direction obtained from the volume data.
Therefore, when the wall motion information is superimposed and displayed on the myocardium represented in the short-axis image according to the conventional technique, a problem arises in which the thickness direction of the myocardium represented in the short-axis image does not coincide with the direction of the vector in the actual wall-thickness direction of the myocardium. As a result, the short-axis image with the wall motion information superimposed does not represent the actual wall motion information of the myocardium represented in the short-axis image. Therefore, in the conventional technique, it is difficult to properly evaluate a wall motion at each part of the myocardium, even if observing the morphology and wall motion information of the myocardium represented in the short-axis image.
Further, according to the method of setting a plane from the central axis toward the myocardium, there is a problem that, depending on the angle of the plane set from the central axis toward the myocardium, the shape of the cardiac cavity represented in an image on the plane significantly differs from the actual shape. For example, there is a problem in which the size of the cardiac cavity represented in the image significantly differs from the size of the actual cardiac cavity.
Furthermore, in setting of the plane from the central axis toward the myocardium, a plane intersecting the myocardium is set with reference to a point on the central axis. Thus, since the plane intersecting the myocardium is set with reference to only the point set on the central axis, it is difficult to set the plane to a desired position of the myocardium. Accordingly, it is difficult to observe a desired cross-section of the myocardium. That is to say, despite observation of a desired cross-section of the myocardium, the plane is set with reference to a point on the central axis set in the cardiac cavity away from the myocardium, and therefore, it is difficult to set the plane to a desired position of the myocardium.